Understand the technical requirements of flue expansion joint, and explain it clearly from type selection to acceptance
Two days ago, a friend who was doing a power plant project called and asked as soon as he opened his mouth, "How should I pick the flue expansion joint? After installing it, there are always problems. Are the technical requirements not thoroughly understood?" I smiled over the phone-this question really can't be explained in a sentence or two. The flue expansion joint looks like a bellows with two flanges, but when it comes to selection and installation, the details can drive people crazy. Today, let's take six steps, break up the practical points from the working conditions to the final acceptance and explain them clearly. If you follow it, it won't overturn.
Step 1: Find out the working conditions of your flue first-temperature, pressure and medium are indispensable
"What type of expansion joint is good?" Tsk, that order is wrong. Without first figuring out what "fairy" working conditions are running in the pipeline, it is useless to choose the expensive model.
Temperature: The flue temperature span is extremely large, from tens of degrees of desulfurization flue gas to thousands of degrees of boiler tail gas. Different temperatures determine the choice of material for expansion joints-metals can typically withstand up to 400-800°C (e.g.Corrugated expansion joint for power station industry), but if you exceed this range, you have to consider lining with insulation or replacing it with non-metallic ones. non-metallicFabric fiber expansion jointTemperature resistance limits are usually lower, but corrosion resistance is better. You have to get the design temperature value and leave another 10-15% margin.
pressure: The flue usually operates at low pressure (several kPa to tens of kPa), but don't think that low pressure can be chosen casually. Although the pressure is small, the stiffness calculation of the bellows cannot be fooled by combining the temperature and pipe diameter. It is recommended to refer to the stiffness and calculation formula of bellows on this site. It is the last word to calculate clearly.
Medium: Flue gas often contains sulfur, moisture and even corrosive gases. At this time, ordinary stainless steel (such as 304) can't stand it, so you have to use 316L or higher corrosion-resistant materials. If the medium contains a large amount of dust, the wear-resistant design of the guide tube must also keep up. Don't forget,Desulfurization flue gas baffle doorThe matching expansion joint is particularly prone to corrosion, and if the material is chosen wrong, it will leak in half a year.
Step 2: Choose the Right Expansion Joint Type – Metallic or Non-Metallic? Rectangular or circular?
After the working conditions are figured out, the next step is to determine the type according to the pipe shape and displacement requirements.
Circular pipe: preference is given to metal corrugated expansion joints, such asUniversal corrugated expansion joint、High temperature axial expansion joint、External pressure single axial expansion jointWait. They have large displacement compensation capability and long life. If space is constrained or multi-directional compensation is required, you can chooseCompound hinge transverse expansion jointOrStraight pipe pressure balanced expansion jointTo avoid additional thrust from the pipe.
Rectangular pipe: It is commonly found in the smoke air duct and dust collector inlet and outlet of power plants. At this timeMetal rectangular expansion jointOrRectangular non-metallic expansion jointIt's the main force. Metallic rectangles are generally used in high-temperature sections (such as economizer outlets), while non-metallic rectangles (ring-belt structures) are more suitable for low-temperature sections with strong corrosion. The non-metallic one has another benefit – easy installation and light weight.
So when did you use rubber? Likerubber compensatorOrRubber PTFE compensatorGenerally used in places with low pressure, normal temperature and high sealing requirements, such as fan outlet. Don't use it in the wrong place, rubber can't stand high temperatures.
Step 3: Technical requirements of core components-how to match guide tube, tie rod and ring belt
This part is real technical work. Whether a flue expansion joint is good or not depends on whether these three accessories are matched correctly.
guide tube: Its function is to guide the airflow smoothly and prevent high-speed smoke from directly washing the bellows. For the specific function, please see the "Specific Function of Expansion Joint Guide Tube" of this site, which has pictures and truth. Remember: the direction of the guide must be consistent with the direction of the medium flow (this is what the arrow on the expansion joint marks), and there must be a gap between the guide and the bellows, otherwise the thermal expansion will squeeze the bellows. The end of the guide tube is best flanged or wear-resistant surfacing to deal with dusty smoke.
tie rod: The tie rod is not intended to hold the expansion joint, but to limit excessive stretching or compression of the bellows. See "Function of Expansion Joint Tie Rod" and "How to Adjust Expansion Joint Tie Rod Nut". When adjusting the tie rod nut, it is generally necessary to reserve the displacement after installation. Screw the nut to the marked position in the cold state, and never lock it. After the installation and pressure test, many tie rod nuts need to be removed or withdrawn to a free state. See the design documents for details. Don't be lazy.
circular belt: This is the core of non-metallic expansion joints. The outer layer of the loop belt is silicone cloth or fluorine rubber cloth, and the inner layer is insulation cotton + stainless steel wire mesh. The technical requirements focus on: interlayer bond strength, temperature resistance grade and flame retardancy. If you are buyingNon-metallic expansion joint (fabric fiber expansion joint)After arrival, be sure to check whether the ring belt is damaged, delaminated or bulged, and check whether the pressure level matches the working condition.
Step 4: Several steps must be checked before installation-model, arrow, pre-stretch one do not leak
The equipment has arrived at the site, so don't rush to get on the crane. Spending half an hour to do three inspections first can save 90% of the follow-up trouble.
- Check model and size: Compare the drawings to see if it isMetal rectangular expansion jointOr something else? Is the flange bolt hole spacing correct? Don't pretend halfway to find out that the holes don't match, and the scene will be awkward.
- Find the direction of the arrow: The expansion joint is generally marked with a flow direction arrow. For the meaning of this arrow, see "The direction of the arrow of the expansion joint refers to". The arrow must be consistent with the flow direction of the pipeline medium. If the guide tube is installed backwards, it will become a resistance member, and the bellows will be worn out quickly.
- Confirm pre-tension/compression status: When installing at room temperature, the expansion joint should be pre-stretched or compressed according to the design displacement. For example, the pipe will have a thermal elongation of 20mm, so compress the expansion joint by 10mm during installation. This value will be specified in the design file. A common mistake in the field is to install it directly, and as a result, the bellows will be pulled or fractured during operation.
Step 5: Hands-on installation-practical points of welding, bolting and baffle door matching
The most error-prone places when installing, I list you against dry.
Welding:If the expansion joint is with a connecting short tube, it is necessary to spot welding and position it first, and then jump welding in sections to avoid the deformation of the bellows caused by heat concentration. During the welding process, the bellows should be covered with a damp cloth to prevent scalding by welding slag. After welding, remove the coating and do non-destructive testing (penetration or magnetic powder). If it leaks, it is a hidden danger.
Bolt tightening:The bolts connected with flanges should be gradually tightened diagonally, and the torque value should be according to the drawing. Don't screw it all at once, otherwise the flange surface will deform and cause leakage. In addition,Flue gas baffle door(e.g.Electric plug-in insulation door、Double-sealed single-axis circular baffle door) When installing the expansion joints on both sides, ensure that the baffle door opens and closes flexibly, and the expansion joints cannot be stuck to the baffle.
Fit clearance:Leave thermal expansion space between the rectangular expansion joint and the adjacent pipe. The common practice is to install limit blocks at both ends of the expansion joint, and remove them after they are installed in place. There is another practical skill: install one end first, then adjust the alignment of the other end and then tighten the bolts. Don't pull hard.
Step 6: Acceptance of trial operation-Has the displacement been measured? Is the leak rate up to par?
It's done when you put it on? Wrong. Acceptance is the time to see the true chapter.
Displacement test:After the pipeline heats up, measure the actual displacement of the expansion joint with a scale or dial indicator to see if it is within the design range. If the displacement exceeds the design value, there may be a problem with the pipe bracket or a failure of the fixing bracket. At this time, stop and adjust, don't fool.
Leakage Rate Test:Flue expansion joints typically require zero leakage (especially with polluting smoke). Check all welds and flange surfaces with soapy water or blowing agent. If there is slight air leakage at the ring belt of non-metallic expansion joint, you can try to tighten the beading bolt; If it still leaks, it means that the ring belt is aging or the beading is uneven, and it must be replaced.
Record in the acceptance documentService life of expansion jointEstimate and next inspection cycle. In general, the design life of metal expansion joints is 8-10 years, and the non-metal ring belts are changed every 3-5 years. However, the actual life is greatly affected by the working conditions. It is recommended to check the ring belt and bellows for cracks and corrosion thinning at least once a year.
After understanding these six steps, the technical requirements of flue expansion joints basically cover their hands. Finally, a reminder: Don't just focus on the parameters in the product manual, the actual working condition is the boss. If you have specific items on hand and are unsure, welcome to look through this siteMetal hose、Non-metallic expansion joint、Flue gas baffle doorAnd other product information, there are a lot of actual combat data for reference. Okay, that's it, let's do it.